1. Field of the Invention
The present invention relates to a wireless communications device and method for use in digital wireless communications systems.
2. Description of the Related Art
Widespread use of cellular phones, pagers, and other mobile communications devices in recent years has produced an explosive increase in the number of users of such devices. On the other hand, there remains difficulty for further growth in that introduction of any new wireless communications system today cannot rid itself of a conundrum of assigning frequency bands not used by existing wireless systems.
It is against this background that the Ultra Wideband (UWB) transmission system is capturing attention as new wireless technology that can make effective use of limited frequency resources. The UWB transmission system basically uses signals made up of pulse strings of very fine pulse widths (for example, 1 ns (nanosecond) or less) to carry out base band transmission.
Also, the UWB is a bandwidth of such order of GHz that a value obtained by dividing the occupied bandwidth by its central frequency (for example, 1 GHz to 10 GHz) comes to approximately 1, and by comparison with the bandwidth used for wireless LAN employing the so-called W-CDMA, cdma2000, and Spread Spectrum (SP) or Orthogonal Frequency Division Multiplexing (OFDM), it occupies an ultra-wide bandwidth.
Further, since the UWB transmission system has a characteristic of low signal power density which makes it difficult to interfere with other wireless systems, it is expected that the UWB transmission system may be used while overlaying frequency bands currently used by the existing wireless system. Still further, it is also expected that its ultra-wide band feature provides a basis of a super high-speed wireless transmitting technique of 100 Mbps level for use in the Personal Area Network (PAN).
Now, as a modulation system using the UWB transmission system, there is, for example, the Pulse Position Modulation (PPM) disclosed in Japanese Patent Laid Open No. 98-508725 and U.S. Pat. No. 6,026,125, which indicates 0/1 information using signals with the pulse generating timing delicately staggered forward or back. As another modulation system, there is proposed the Bi-phase Modulation arranged to indicate 0/1 information by changing a pulse phase.
The Direct Spread (DS) system, one type of a spread spectrum (SS) system, is designed to spread occupied bandwidth and transmit information signals by multiplying the information signals by a random code sequence called the Pseudo Noise (PN) code. On the receiving end, the received spread information signals are multiplied by the PN code to de-spread so as to reproduce the information signals.
The UWB transmission method is a system that increases the spread rate of the information signals to a upper limit. Since the signals spread by the UWB transmission system only have power below the noise level in every frequency range, the communications system based on the UWB transmission system is advantageous in that its coexistence with other communications systems is relatively easy.
An example of transmission using the UWB system is shown in FIG. 10. Inputted information 1001 is spread by a spreading code sequence 1002. Depending on a configuration of system that uses the UWB system, this step of spreading code sequence multiplication may be omitted.
An information signal 1003 subjected to spread spectrum operation is modulated by using an impulse signal (wavelet pulse) 1004 according to the UWB system. For the modulation system, the Pulse Position Modulation (PPM), phase modulation, amplitude modulation and the like may be utilized.
Since the impulse signals used in the UWB system consist of very fine (narrow) pulses, a very wide band is used in terms of frequency spectrum, thereby leading to the inputted information signals having power under the noise level in every frequency range.
Although a received signal 1005 is mixed in noise, it can be detected by calculating a correlation value between the received signal and the impulse signal. Further, since signal spreading is performed in a number of systems, many impulse signals are transmitted with respect to 1 bit of transmitting information, thereby making it possible to integrate a receiving correlation value 1006 of the impulse signal further for the length of the spreading code sequence and thus making it easier to detect transmission signals much easier.
FIG. 11 shows a configuration example of a wireless communications terminal using the UWB system. A wireless terminal 1101 is configured to have elements 1111-1114 for transmitting, elements 1103-1108 for receiving, further a transmit/receive timing control section 1109, an RF section 1102, and a central control unit 1110.
In transmitting, after information to be transmitted is processed for information source coding 1114 and channel coding 1113, it is stored in an information buffer 1112, inputted into a pulse circuit generator 1111, and transmitted at a proper timing.
Also, in receiving, calculation of the correlation value between a received signal and a UWB impulse signal is performed, its output being integrated for the number of pulses of 1 bit of transmission signals 1104. Thereafter, the output of the integrated value is subjected to A/D conversion 1105, stored in a receiving buffer 1106, and the information stored in the receiving buffer is decoded via channel decoding 1107 and information source decoding 1108.
Further, in the RF section 1102, there are performed processes such as transmit/receive changeover, transmit/receive filter processing, and signal amplification.
FIG. 12 is a block diagram showing a more detailed configuration of a receiver according to the UWB communications system. FIG. 13 is a diagram showing correlation characteristics at a principal part 1609 of a timing synchronous circuit having a configuration of the so-called Delay Lock Loop (DLL) of the receiver shown in FIG. 12.
A radio signal is received by an antenna 1201, and the received signal, after any unnecessary component is eliminated in a band pass filter 1202, is outputted to multipliers 1207, 1213, and 1210.
A spreading code generator 1204 outputs a spreading code sequence (the same spreading code sequence as the spreading code sequence used in the transmitter shown in FIG. 10) at a frequency of a synthesizer 1203 to a pulse generator 1205. In the pulse generator 1205, a pulse is generated, and at the same time, the spreading code sequence outputted from the spreading code generator 1204 is overlaid on the pulse and outputted to delay units 1206 and 1212 and also to a multiplier 1210.
In the delay unit 1206, the pulse, on which the spreading code sequence was overlaid, is delayed by a width of ½ pulse and outputted to 1207. Also, In the delay unit 1212, the pulse, on which the spreading code sequence was overlaid, is delayed by a width of 1 pulse and outputted to the multiplier 1213.
Consequently, the pulse, on which the spreading code sequence was overlaid for demodulating transmitting data, is multiplied by the received signal and despread processing is performed. Also, in the multiplier 1210, the pulse, on which the spreading code sequence was overlaid, is multiplied by the received signal at a timing of a ½ pulse width in advance of the output of the delay unit 1206, and despread processing is performed.
The result of multiplication of the multiplier 1207 is outputted to an integrator 1208 to be integrated thereby, and outputted as receiving data. The result of multiplication of the multiplier 1210 is outputted to an integrator 1211, integrated thereby, and outputted to a differentiator 1215 (1302 in FIG. 13). The result of multiplication of the multiplier 1213 is outputted to an integrator 1214, integrated thereby, and outputted to a differentiator 1215 (1301 in FIG. 13).
A difference between the output of the integrator 1211 and the output of the integrator 1214 (1303 in FIG. 13 in a solid line) is calculated at the differentiator 1215, and the result is outputted to a loop filter 1216. As apparent from FIG. 13, with respect to a phase shift (transverse axis), the output
In the synthesizer 1203, it is so controlled that if the loop filter 1216 output is positive, the generated phase of the spreading code sequence is slightly delayed, and if it is negative, the generated phase of the spreading code sequence is slightly advanced. This operation enables the loop filter 1216 output (difference) to become zero, and the pulses, on which the spreading code sequence to be supplied to the multiplier 1207 was overlaid, fall in agreement with the phase of the received signal, thus maximizing the despread output.